Journal of Coastal Research SI 36 333-339 (ICS 2002 Proceedings) Northern Ireland ISSN 0749-0208

Seafloor Features and Characteristics of the Black Ledges Area, , , USA.

Allen M. Gontz, Daniel F. Belknap and Joseph T. Kelley

Department of Geological Sciences, 111 Bryand Global Science Center, University of Maine, Orono, Maine, USA04469-5790. [email protected], [email protected], [email protected]

ABSTRACT The Black Ledges, a series of islands, shoals, and ledges in East Penobscot Bay, Maine, was mapped with digital sidescan sonar and shallow marine seismic reflection equipmentA total of 38 km2 of sidescan and 600 km of seismic data was collected during four cruises in 2000-2001. The sidescan sonar reveals a surficial geology dominated by muddy sediments with frequent, patchy outcrops of gravel and minor amounts of bedrock. There are seven large concentrations of pockmarks with populations totaling over 3500 in the areas of muddy sediments. Generally circular, pockmarks range in size from five to 75 meters in diameter and up to eight meters deep. Calculations show over 2 x 106 m3 of muddy sediment and pore water were removed from the system during pockmark formation. Seismic data reveal a simple stratigraphy of modern mud overlying late Pleistocene glaciomarine sediment, till and Paleozoic bedrock. Seismic data indicate areas of gas-rich sediments and gas- enhanced reflectors in close association with pockmarks, suggesting methane seepage as a cause of pockmark formation. Pockmarks are alsorecognized in areas lacking evidence of subsurface methane accumulations adding further validity to the late stage of development for the field. Elliptical pockmarks, found in nearly 40 m of water, show modification by currents and degradation of the pockmark form. This suggests depletion of methane and a late stage of development. A more intensive investigation of the area, including coring and high-resolution geophysics is currently in progress. ADDITIONALINDEXWORDS: Pockmarks, sidescan sonar

INTRODUCTION Pockmarks release methane into the water column and Habitat mapping is currently an area of intense focus by potentially into the atmosphere. Methane is a potent the Geological Survey, National Oceanic and greenhouse gas. It traps infrared radiation up to 21 times Atmospheric A s s o c i a t i o n ’s National Marine Fisheries more readily than carbon dioxide (LAMMERS et al., 1995). Service, and state-level equivalents. In order to properly Not only does this release have the potential to alter the assess seafloor habitat, knowledge of the surficial geology global climate system, but it also provides a readily is essential. Geophysical tools such as sidescan sonar and available source of labile carbon for biological activity. The shallow seismic reflection profiling permit the necessary contribution of the marine system to the global carbon cycle observations. is unquantified (LAMMERS et al., 1995). Clearly, a better Penobscot Bay, a large, productive in the Gulf of understanding of these features, the relationships between Maine has been a focus area for such mapping (Figure 1). biological and physical processes, and their effects on the Previous studies in Penobscot Bay, specifically the sub- marine system is needed. embayment of , have revealed a dynamic The Black Ledges area, the focus of this study, is also seafloor environment dominated by football stadium-sized within Penobscot Bay, but sufficiently removed from features known as pockmarks. KING and MACLEAN Belfast Bay (Figure 1) to be influenced by different water (1970) first described these features on the Scotian Shelf. masses and current regimes (XUE et al., 1999). Workers They are believed to form from pressurized pore fluid from the Maine Geological Survey and the Department of escape (HOVLAND and JUDD, 1988). While these Geological Sciences at the University of Maine collected features are best studied in one part of Penobscot Bay, they several reconnaissance lines in the area between 1984 and also occur throughout the region (BARNHARDT a n d 1999. These few lines showed a potential for pockmarks KELLEY, 1995; FADER 1991). and prompted a more intensive investigation.

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Figure 1. Location of Penobscot Bay. The large box in the center of the figure locates Penobscot Bay. The inset map shows the location of Maine’s coast within the region. Penobscot Bay is the largest estuary along Maine’s coast. Box A shows location of the Black Ledges area, the focus of this study and Box B shows the location of Belfast Bay, which also hosts pockmarks. NH is New Hampshire, NB is New Brunswick, Canada.

This paper reports on the first portion of a major research August 2000 and Applied Acoustics Engineering’s digital effort focused on shallow-water, estuarine pockmarks. This system in later cruises), were towed simultaneously. portion involves baseline surficial and subsurface mapping Sidescan sonar data were collected at 100kHz and 500kHz of a pockmarked area and the surrounding seafloor to and seismic data were collected at 700-2000 Hz. ATriton- identify features and characteristics of the area. T h e Elics International topside processing unit was used to ultimate goal is to determine the source of methane, the acquire and post-process the data for both digital systems. origin and age of the source materials, their role in sea-level Sidescan data were collected and processed with Isis changes, potential mechanisms for formation, life cycles, Sonar v4.54 and Delph Map. All data were slant-range e ffect on the global climate system, and effects on corrected. Digital seismic data were collected and sedimentation and seafloor habitat. In this report we processed with Delph Seismic and Seismic GIS. Analog present new data from a series of newly discovered seismic data were analyzed without computer technology. pockmark fields. More importantly, we develop evidence Surficial and subsurface mapping and spatial analysis were for a model of pockmark evolution, including, for the first conducted with A r c View and ArcInfo geographic time, the late stages of development and senescence. information systems software. Navigation was supplied via differential global positioning system and converted to METHODS Universal Transverse Mercator projection for use. The Capn, a digital navigation package produced by Nautical The seafloor was mapped with sidescan sonar and Technologies, was used for cruise planning, cruise shallow seismic reflection profiling equipment during four navigation, autopilot interface, and 30-second position daylong cruises, three in August 2000 and one in January logging. 2001, in the Black Ledges area. The two systems, an The shallow water (<40 m) allowed for collection of Edgetech DF1000 digital, dual-frequency sidescan sonar sidescan data at 100 m range. The fish was towed about towfish and a surface-towed boomer seismic system four to six meters deep at between four and six knots. (Ocean Research Engineering’s analog GeoPulse during

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RESULTS Natural gas, methane, is found within the section at many places. It occurs as areas of large-scale acoustic wipeout or A total of 38 km2 of seafloor was imaged with sidescan gas-enhanced reflectors (Figure 4) (YUAN et al., 1992; sonar and over 600 km of seismic data were collected F L O O D G ATE and JUDD, 1992; FADER, 1991). during four cruises. A complete sidescan sonar mosaic was Stratigraphically, the methane occurs within the Holocene created for the area (Figure 2). muds or along the Holocene-Pleistocene unconformity. A stratigraphic framework of Penobscot Bay was Pockmarks have been observed over areas of acoustic developed from seismic data and supported with sediment wipeout and gas-enhanced reflectors and both indicators of cores. (BARNHARDT et al., 1997; BELKNAP and SHIPP, methane have been observed without associated pockmarks. 1991; KNEBEL and SCANLON, 1985). Seismic records The enhanced reflectors suggest methane is migrating along show acoustic basement as bedrock, overlain by till, the unconformity. Methane is hypothesized to be the fluid glaciomarine sediments, capped with Holocene mud. Till involved in pockmark formation (KELLEY et al., 1994). was not present in all locations and was thickest on The degree of backscatter from the sidescan data was southeastern slopes. Two major unconformities, the basal interpreted as a proxy for bottom sediment type. Areas of erosional unconformity created during a sea-level lowstand, high backscatter, such as gravel and rock, appear dark gray and the ravinement unconformity formed during the latest to black while areas of low backscatter, such as mud, appear transgression, truncate the glaciomarine sediments light gray to white. The surficial sediment type was mapped (BARNHARDT et al., 1997; BELKNAPand SHIPP, 1991). following a sixteen-part scheme with four end members of mud, sand, gravel, and rock and 12 intermediates (BARNHARDT et al., 1996; 1998).

Figure 2. Detailed map of the Black Ledges area. The outlined area shows the limits of sidescan sonar coverage detailed in Figure 4. Shaded polygons within the limits of sidescan coverage represent the seven pockmark fields. Line A-A’ is seismic line PB01-26 used in Figure 3. Box A is the location of sidescan sonar data for Figure 5. Box B is the location of sidescan sonar data for Figure 6.

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Figure 3. Seismic line A-A’. Location of seismic line is shown in Figure 2. Upper panel shows uninterpreted data, lower panel is interpreted. Gas-enhanced reflectors (GER) originate from large areas of acoustic wipeout (NG) suggesting migration along the reflector. Pockmarks, depressions on the seafloor, (PM) occur directly over NG and GER. Other units include till (T), bedrock (BR), glaciomarine (GM) and Holocene mud (M). VE=14x.

Figure 4. Sidescan sonar mosaic of the Black Ledges area. A mosaic was created from 38 km2 of sidescan sonar data. Light areas, low backscatter, are interpreted as fine-grained sediments (muds) and dark areas, high backscatter, are interpreted as hard surfaces (rock and gravel). Pockmarks are associated with fine-grained, low backscatter areas. Box A locates Figure 5 and Box B locates Figure 6.

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Figure 5. Circular Pockmarks of the Black Ledges. Pockmarks, generally circular in form occur in large concentrations around the Black Ledges. The circular form indicates the pockmarks are active. Refer to Figure 4 for location.

Figure 6. Elliptical pockmarks of the Black Ledges. Elliptical pockmarks indicate that the processes that form and maintain pockmarks (gas-escape) are no longer the dominant processes. The elliptical shape likely arises from scouring by currents. These pockmarks are orientated 045° (true) azimuth and occur sub-parallel to the 37 m isobath. They are only found deeper than the 37 m isobath. Elliptical pockmarks represent a previously unrecognized late stage of development.

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area; rock crops out on only 1% of the study area. Pockmarks occur in other regions of Penobscot Bay, most notably in Belfast Bay (ROGERS, 1999; KELLEY et al., 1994; KNEBEL and SCANLON, 1985). These features are attributed to methane escape from the seafloor. Seismic lines from the area show shallow subsurface acoustic masking and gas-enhanced reflectors originating in large areas of acoustic wipeout in close proximity to pockmarks, suggesting that methane is the fluid for pockmark excavation. Pockmarks frequently occur over gas-enhanced reflectors, suggesting methane is migrating from areas of acoustic wipeout. The pockmarks are only observed in muddy sediments. Other researchers (e. g. FADER, 1991; HOVLAND and JUDD, 1988) have presented data suggesting fine-grained sediments are needed to trap migrating methane to allow a pressurized reservoir to develop. An analysis of captured in situ pore fluids confirms methane is the gas (H. A. CHRISTIAN, personal Figure 7. Distribution of surficial units in the Black Ledges. communication, 2000). The sidescan sonar mosaic was interpreted for The occurrence of elliptical pockmarks suggests surficial geology. A 16-unit classification scheme was subsequent modification by current activity. The elliptical used (BARNHARDT et al., 1998). The area is clearly nature also suggests that current activity is more important dominated by muddy units with subordinate gravel in the development of the form of these pockmarks than units. Rock units are unimportant and sandy units methane escape. This is validated by the present lack of were not observed at the mapping scale. methane (acoustic masking and gas-enhanced reflectors) imaged with seismic devices in the vicinity of the elliptical pockmarks. This implies that the reservoir of natural gas, or the generation potential of organic-rich sediments is The majority of the area is covered with mud-dominated exhausted; seismic data from areas with circular pockmarks classes (about 64%). Gravel (35%) and rock (1%) cover the show larger numbers of gas-enhanced reflectors and large- remainder of the seafloor. Sand is not present at this map scale acoustic wipeout. Seasonal variability in subsurface scale (Figure 5). natural gas concentrations (MARTENS et al., 1998) or Sidescan sonar images reveal seven large concentrations alignment of survey lines are possible alternate explanations of pockmarks (Figure 6). The seven fields contain 3528 for the lack of gas in the area. pockmarks ranging in size from five to 75 meters in The sediments and stratigraphy of the entire area, which diameter and up to eight meters deep (Figure 7). Pockmarks comprises thin Holocene sediments, gravel, and boulder are only associated with muddy sediments. Rough lags developed on glaciomarine sediments suggests an calculations based on a method developed by ROGERS overall erosive environment compared to one of sediment (1999) suggest that over 2.10x106 m3 of seafloor muddy accumulation that characterizes pockmark formation in sediment and pore water were removed. Belfast Bay. The pockmark formation and maintenance Pockmarks in all but one field appear unmodified by process accentuates the erosion of muddy sediments. Over currents or slumping. Features in the northern most area 2 x 106 m3 of muddy sediments and pore water were mapped appear elliptical with their long axis orientated at removed from the area by pockmark formation. Thus, about 045° and sub-parallel to the 37 m contour (Figure 6). pockmarks are an effective method of redistributing All of these features occur deeper than the 37 m contour. sediments in . Data from other regions of Penobscot Bay show that the seafloor can be resurface in DISCUSSION less than two years, filling depressions upwards of 25 m in B A R N H A R D T et al. (1996) initially interpreted the diameter and five meters deep, in areas with pockmarks Black Ledges as a mud and rock-dominated system. Their (GONTZ, 2001). interpretation was produced from bathymetric analysis and only a few survey lines in the area. New data presented in CONCLUSIONS this study confirms their interpretation of a mud-dominated The Black Ledges area is a mud-dominated system that seafloor, but does not identify rock as a major surficial unit. hosts seven major pockmark fields. In one of these fields, Our data show that mud covers 64% and gravel 35% of the

Journal of Coastal Research, Special Issue 36, 2002 Gontz, Belknap and Kelley 339 the pockmarks are elliptical in form, suggesting that current J. B. ANDERSON and G. M. ASHLEY (eds.), Glacial- action has replaced methane venting as the major Marine Sedimentation: Paleoclimactic Significance, pockmark-related process. This and the rest of the Geological Society of America Special Paper 261, 137- pockmark fields occur in thin Holocene sediments with few 157. zones of large-scale acoustic wipeout and gas-enhanced FADER, G. B. J., 1991. Gas-related sedimentary features reflectors. This suggests the subsurface methane supply is from the eastern Canadian continental shelf. Continental depleting or is exhausted. Both of these observations point Shelf Research, 11, 8-10, 1123-1153. to a previously unrecognized late stage of pockmark field FLOODGATE, G. D. and JUDD, A. G., 1992. Origins of evolution. This is in great contrast to pockmark systems shallow gas. Continental Shelf Research, 12, 10, 1145- recognized in other estuaries which show large amounts of 1156. gas-charged sediments and pockmarks (KELLEY et al., GONTZ, 2001. Evolution of Seabed Pockmarks in 1995; FADER, 1991; HOVLAND and JUDD, 1988). Penobscot Bay, Maine. Unpublished Masters Thesis, The volume of gas is unknown at this time. Future University of Maine, Orono, Maine, 91pp. research is aimed at determining the source of methane, the HOVLAND M. A. and JUDD, A.G., 1988. S e a b e d age and depositional environment of the source material, Pockmarks and Seepages. Graham and Tr o t m a n , and the potential for methane generation. In addition, the London, 144p. flux of methane from estuarine pockmarks to the water KELLEY, J. T., DICKSON, S. D., BELKNAP, D. F., column and atmosphere will be investigated. BARNHARDT, W. A., and HENDERSON, M., 1994. Giant sea-bed pockmarks: evidence for gas escape from ACKNOWLEDGEMENTS Belfast Bay, Maine. Geology, 22, 59-62. KING, L. H. and MACLEAN, B., 1970. Pockmarks on the We acknowledge Maine Sea Grant (R/CE-235), National Scotian Shelf. Geological Society of America Bulletin, Science Foundation (OCE-9977367), and the National 81, 3141-3148. Oceanic and Atmospheric Association (shiptime support) KNEBEL, H. J. and SCANLON, K. M., 1985. Sedimentary are acknowledged for supporting this research. We thank framework of Penobscot Bay, Maine. Marine Geology, Tony Codega, captain of the RV Friendship, for hours at sea 65, 305-324. and his willingness to lend a hand when needed. Colleagues LAMMERS, S., SUESS, E., and HOVLAND, M., 1995. A at the University of Maine and Maine Geological Survey large methane plume east of Bear Island (Barents Sea): also assisted with field and lab work. implications for the marine methane cycle. G e o l Rundsch, 84, 59-66. LITERATURE CITED MARTENS, C. S., ALBERT, D.B., and ALPERIN, M. J., BARNHARDT, W.A. and KELLEY, J. T., 1995. Carbonate M. J., 1999. Biogeochemical processes controlling accumulation on the inner continental shelf of Maine: a methane in gassy coastal sediments – part 1 a model consequence of later Quaternary glaciation and sea-level coupling organic matter flux to gas production, oxidation change. Journal of Sedimentary Researc h, A65, 1, and transport. Continental Shelf Research, 18, 1741- 195-207. 1770. BARNHARDT, W. A., BELKNAP, D. F., and KELLEY, J. ROGERS, 1999. Mapping of Subaqueos, Gas-Related T., 1997. Stratigraphic evolution of the inner continental Pockmarks in Belfast Bay, Maine Using GIS and Remote shelf in response to late Quaternary relative sea-level Sensing Te c h n i q u e s, Unpublished Masters T h e s i s , change, northwest Gulf of Maine. Geological Society of University of Maine, Orono, Maine, 139pp. America Bulletin, 109, 5, 612-630. XUE, H., XU, Y., BRROKS, D., PETTIGREW, N., and BARNHARDT, W. A., BELKNAP, D. F., KELLEY, A. R., WALLINGA, J., 1999. Modelling the circulation in KELLEY, J. T., and DICKSON, S. M., 1996. Surficial Penobscot Bay, Maine. Estuarine and Coastal Modeling geology of the Maine inner continental shelf: Rockland Proceedings of the Conference of American Society of to Bar Harbor, Maine Geological Survey, Geologic Map Civil Engineers, 1112-1127. 96-10, scale 1:100,000. Augusta, Maine. YUAN, F., BENNELL, J. D., and DAVIS, A. M., 1992. BARNHARDT, W. A., KELLEY, J. T., DICKSON, S. M., Acoustic and physical characteristics of gassy sediments and BELKNAP, D. F., 1998. Mapping the Gulf of Maine in the western Irish Sea. Continental Shelf Research, 12, with Sidescan Sonar: a complex new bottom-type 10, 1121-1134. classification for complex seafloors. Journal of Coastal Research, 14, 2, 646-659. B E L K N A P, D. F. and SHIPP, R. C., 1991. Seismic stratigraphy of glacial-marine units, Maine inner shelf. In

Journal of Coastal Research, Special Issue 36, 2002